M(3-x)(NH4)(x)CrO8 (M = Na, K, Rb, Cs): a new family of Cr5+-based magnetic ferroelectrics

Upon consideration of the hydrogen-bonding properties of the NH(4)(+) cation, we synthesized a new class of compounds, M(3-x)(NH(4))(x)CrO(8) (M = Na, K, Rb, Cs). These magnetic compounds with the simple 3d(1) ground state become ferroelectric. X-ray studies confirmed that the phase transition involves a symmetry change from I42m to Cmc2(1) to P1. The transition temperature depends linearly on the composition variable x. The transitions are of the order-disorder type, with N-H···O bonding playing the central role in the mechanism. Extension of this idea to the introduction of ferroelectricity in several other classes of materials is suggested.